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We report an investigation into the impact of applying a large reverse bias voltage during current–voltage (J-V) sweeps on the degradation rate of solar cells based on poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6-6]-phenyl C71-butyric acid methyl ester (PC70BM), under air mass 1.5 simulated solar irradiation. The degradation rate was found to be significantly slower when devices were characterized using frequent J-V sweeps with a large reverse bias component (-7 V to 1 V), with a decrease in power conversion efficiency of 24% after ∼ 100 hrs of light exposure. In contrast, devices scanned from −1 V to 1 V degrade by 72% in the same time period. The decay in the photovoltaic performance was found to be related to an increase in the series resistance and a decrease in the shunt resistance of the devices over time. Further characterization of the bulk heterojunction layer under irradiation conditions indicates that this behavior might be caused by the decreasing mobility of the charge carriers due to the formation of the defects or traps in the layer, induced by the photo-oxidation process. The frequent application of a large reverse bias sweep most likely helps de-trap the charge carriers and slows down the device degradation. Impedance analysis indicated that during the decay of PCDTBT:PC70BM devices the bulk layer became more resistive with time, and applying large and frequent reverse bias sweeps during testing could significantly slow down this process. The charge carrier extraction time was found to increase from 7 μs to more than 400 μs after photo-degradation. These findings demonstrate that repeated application of a reverse bias voltage can affect device degradation, and therefore that the specific details of J-V characterization should be reported alongside device lifetime claims.

Purpose Stationary computed tomography (s-CT) conceptually offers several advantages over existing rotating gantry-based CT. Over the last 40 yr, s-CT has been investigated using different technological approaches. We are developing a s-CT system specifically for head/brain imaging using carbon nanotube (CNT)-based field emission x-ray source array technology. The noncircular geometry requires different assessment approaches as compared to circular geometries. The purpose of the present study is to investigate whether the CNT source array meets the requirements for stationary head CT (s-HCT). Methods Multiple prototype CNT x-ray source arrays were manufactured based on the system requirements obtained from simulation. Source characterization was performed using a benchtop setup consisting of an x-ray source array with 45 distributed focal spots, each operating at 120 kVp, and an electronic control system (ECS) for high speed control of the x-ray output from individual focal spots. Due to the forward-angled geometry of the linear anode, the projected focal spot shape is expected to vary at wide angle views. A pinhole method was implemented to determine the effective focal spot size (FSS) in the imaging plane at a range of angular viewpoints with a flat panel detector. The output spectrum and half value layer (HVL) were also evaluated for a range of viewing angles to characterize the beam quality across the fan-beam. Dosimetry was performed on a simulated scan to evaluate total exposure. Results The prototype CNT x-ray source array demonstrated adequate specifications for a s-HCT imaging machine. The source array was operated at 120 kVp with long-term stability over a full year of regular laboratory use. Multiple cathode current measurements were used to confirm submicrosecond accuracy with regards to exposure time and subsequently dose control. All 45 focal spots were measured with an average value of 1.26 (±0.04) mm × 1.21 (±0.03) mm (equivalent to IEC 1,0). The x-ray spectrum was found to be appropriately filtered based on sources used in existing rotary CT systems. A stable and reliable output of 0.04 mAs per emitter and a resulting dose of 0.015 mGy per projection were observed over several months of rigorous phantom imaging. Dose per projection was regulated by the ECS and measured with ±0.5% tolerance. Conclusions The CNT x-ray source array was found to meet the requirements for the proposed stationary head CT scanner, with regard to FSS, beam quality, and dose precision. The remaining challenges are related to the overall system design of a nonrotating CT scanner with distributed sources. The next phase of the project will incorporate multiple CNT source arrays with multirow detectors in a proof-of-concept study and analysis of a fully functional s-HCT system.

Biosensing applications are driving an increase in demand for low-cost photodetectors with sensitivity between 1000 and 1400 nm. Recently, the sensitivity of Ag2Se colloidal quantum dot based devic...

Cone beam computed tomography (CBCT) is now widely used in dentistry and growing areas of medical imaging. The presence of strong metal artifacts is however a major concern of using CBCT especially in dentistry due to the presence of highly attenuating dental restorations, fixed appliances, and implants. Virtual monoenergetic images (VMIs) synthesized from dual energy CT (DECT) datasets are known to reduce metal artifacts. Although several techniques exist for DECT imaging, they in general come with significantly increased equipment cost and not available in dental clinics. The objectives of this study were to investigate the feasibility of developing a low-cost dual energy CBCT (DE-CBCT) by retrofitting a regular CBCT scanner with a carbon nanotube (CNT) x-ray source with dual focal spots and corresponding low-energy (LE) and high-energy (HE) spectral filters. A testbed with a CNT field emission x-ray source (NuRay Technology, Chang Zhou, China), a flat panel detector (Teledyne, Waterloo, Canada), and a rotating object stage was used for this feasibility study. Two distinct polychromatic x-ray spectra with the mean photon energies of 66.7keV and 86.3keV were produced at a fixed 120kVp x-ray tube voltage by using Al+Au and Al+Sn foils as the respective LE and HE filters attached to the exist window of the x-ray source. The HE filter attenuated the x-ray photons more than the LE filter. The calculated post-object air kerma rate of the HE beam was 31.7% of the LE beam. An anthropomorphic head phantom (RANDO, Nuclear Associates, Hicksville, NY) with metal beads was imaged using the testbed and the images were reconstructed using an iterative volumetric CT reconstruction algorithm. The VMIs were synthesized using an image-domain basis materials decomposition method with energy ranging from 30 to 150keV. The results were compared to the reconstructed images from a single energy clinical dental CBCT scanner (CS9300, Carestream Dental, Atlanta, GA). A significant reduction of the metal artifacts was observed in the VMI images synthesized at high energies compared to those from the same object imaged by the clinical dental CBCT scanner. The ability of the CNT x-ray source to generate the output needed to compensate the reduction of photon flux due to attenuation from the spectral filters and to maintain the CT imaging time was evaluated. The results demonstrated the feasibility of DE-CBCT imaging using the proposed approach. Metal artifact reduction was achieved in VMIs synthesized. The x-ray output needed for the proposed DE-CBCT can be generated by a fixed-anode CNT x-ray source.

The development of printable and air-stable unipolar n-type semiconductors remains a critical issue in the field of organic electronics in view of n-type transistors being indispensable components in the manufacture of low-power-consumption complementary (CMOS type) integrated circuits. In this work, we report on two dicyanomethylene-substituted diketopyrrolopyrrole derivatives with different alkyl side chains (DCM-DPP-C13 and DCM-DPP-C16) for n-type transistors. Compared with DCM-DPP-C16, the side chain branching point in DCM-DPP-C13 was moved further away from the conjugated backbone. Both materials showed promising electron mobility above 0.1 cm2/VS in air with remarkably low threshold voltages (−3 V – 2 V) on spin-coated bottom-contact bottom-gated devices on silicon substrates. Contrary to our original expectation, the average electron mobility of DCM-DPP-C16 (0.2 cm2/VS) was higher than that of DCM-DPP-C13 (0.12 cm2/VS) due to the bigger crystalline domains in the thin film of the former compound. We investigated the inkjet printability of these two compounds as well, and bottom-contact top-gated transistors were successfully fabricated on flexible PET substrates. Widely used Cytop was employed as the gate dielectric. Ag gate electrodes were readily inkjet printed on the gate dielectric by using a thin layer of Nafion as an adhesion promoter. DCM-DPP-C16 demonstrated an excellent inkjet-ability with good uniformity and reproducibility. An average electron mobility around 0.1 cm2/VS was achieved in air. This is an important step toward the fabrication of large-area organic CMOS logic circuits with low cost.

Large area processing of organic photovoltaic cells and modules is a key step towards commercialization. At the same time, the scale-up of fabrication entails careful evaluation and selection of solvents, and optimization of processing parameters. It has been proven that using proper processing additives in polymer:PCBM bulk-heterojunction solution is one of the most effective strategies to improve the performance of organic photovoltaic cells. Here we report a new transition solvent strategy of using non-halogenated solvents to fabricate PDTSTPD: PC71BM based active layers. This strategy uses a combination of miscible good main solvent such as 1,2,4-trimethylbenzene and coadditives such as diphenyl ether (DPE) and 1-methylnaphtalene (MeN) for middle molecular weight PDTSTPD, or 1,8-diiodooctane (DIO) and MeN for high molecular weight PDTSTPD to tune the nano-scale morphology of the active layer during the film drying process. Inverted organic photovoltaic cells with an active area of 1cm2 on PET substrate fabricated by a blade coating process using the formulated ink have demonstrated a power conversion efficiency up to 5 %.

Purpose Technological advancements in dental radiography have improved oral care on many fronts, yet diagnostic efficacy for some of the most common oral conditions, such as caries, dental cracks and fractures, and periodontal disease, remains relatively low. Driven by the clinical need for a better diagnostic yield for these and other dental conditions, we initiated the development of a stationary intraoral tomosynthesis (s-IOT) imaging system using carbon nanotube (CNT) x-ray source array technology. Here, we report the system characterization and preliminary imaging evaluation of a clinical prototype s-IOT system approved for human use. Methods The clinical prototype s-IOT system is comprised of a multibeam CNT x-ray source array, high voltage generator, control electronics, collimator cone, and dynamic digital intraoral detector. During a tomosynthesis scan, each x-ray source is operated sequentially at fixed, nominal tube current of 7 mA and user-specified pulse width. Images are acquired by a digital intraoral detector and the reconstruction algorithm generates slice information in real time for operator review. In this study, the s-IOT system was characterized for tube output, dosimetry, and spatial resolution. Manufacturer specifications were validated, such as tube current, kVp, and pulse width. Tube current was measured with an oscilloscope on the analog output of the anode power supply. Pulse width, kVp, and peak skin dose were measured with a dosimeter with ion chamber and high voltage accessory. In-plane spatial resolution was evaluated via measurement of MTF and imaging of a line pair phantom. Spatial resolution in the depth direction was evaluated via artifact spread measurement. The size of the collimated radiation field was evaluated for compliance with FDA regulations. A dental phantom and human specimens of varying pathologies were imaged on a clinical 2D intraoral imaging system as well as s-IOT for comparison and to explore potential clinical applications. Results The measured tube current, kVp, and pulse width values were within 3% of the set values. A cumulative peak skin dose of 1.12 mGy was measured for one complete tomosynthesis scan using a 50-ms pulse per projection view. Projection images and reconstruction slices revealed MTF values ranging from 8.1 to 9.3 cycles/mm. Line pair imaging verified this result. The radiation field was found to meet the FDA requirements for intraoral imaging devices. Tomosynthesis reconstruction slice images of the dental phantom and human specimens provided depth resolution, allowing visibility of anatomical features that cannot be seen in the 2D intraoral images. Conclusions The clinical prototype s-IOT device was evaluated and found to meet all manufacturer specifications. Though the system capability is higher, initial investigations are targeting a low-dose range comparable to a single 2D radiograph. Preliminary studies indicated that s-IOT provides increased image quality and feature conspicuity at a dose comparable to a single 2D intraoral radiograph.

Air-stable solution-processable unipolar n-type organic semiconductors are highly desired in the field of organic thin film transistors as n-type transistors are indispensable components in low-power-consumption complementary integrated circuits. The present work seeks to address this issue and describes the synthesis of two dicyano-substituted bis(2-oxoindolin-3-ylidene)benzodifurandione (BOIBDD) derivatives with engineered alkyl side chains for n-type transistors. Two kinds of side chains are used in this study: 2-hexyldecyl (C₁₆H₃₃) and 3-hexylundecyl (C₁₇H₃₅). Surprisingly, the small structural difference between side chains induced remarkable differences in the physical properties and electronic performance of the BOIBDD derivatives. In addition, cyano substitution into the backbone of BOIBDD derivatives was shown to be effective at lowering the HOMO and LUMO energy levels of BOIBDD derivatives to obtain unipolar n-type materials. Compared with fluorine substitution, cyano substitution was readily realized by a simple and straightforward reaction using cheap CuCN as a cyanation reagent. Both atomic force microscopy (AFM) and X-ray diffraction (XRD) studies have confirmed that the BOIBDD derivative with 3-hexylundecyl side chains tended to form highly crystalline structures in the solid state. More interestingly, thermal annealing of this compound at a temperature as low as 75 °C, well below its melting point of 305 °C, can still further improve its crystallinity and thus significantly increase its electron mobility by 43 times to 0.1 cm² V⁻¹ s⁻¹ in air. The insight gained from this study will shed some light on the design of new air-stable high-performance n-type organic semiconductors.

As we move toward roll-to-roll processing of organic photovoltaics (OPVs), it isimportant to validate the process and the ink formulations. A poly[[9-(1-octyl-nonyl)-9Hcarbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7diyl2,5thiophenediyl]:[6,6]-phenyl C71 butyric acid methyl ester (PCDTBT:PC₇₁BM)-based OPV with an increase in power conversion efficiency (PCE) from4.6% up to 5.4% by incorporating bis(tri-hexylsiloxy) silicon phthalocyanine((3HS)2-SiPc) NIR absorbing ternary additives on indium tin oxide (ITO)/poly-ethyleneterapthalate (PET)flexible substrate with active area of 1 cm² undersimulated AM 1.5G one sun irradiation. Maximum PCE of the ternary OPVs wasobtained with 5 wt% of (3HS)₂-SiPc added when processing the active layer byspin coating, whereas the addition of 10 wt% of (3HS)2-SiPc is required for themaximization of PCE when processing thefilms by blade coating. The importanceof processing conditions when optimizing the concentration of ternary additivesin bulk heterojunction (BHJ) OPVs is demonstrated.

Background The assessment of apparent diffusion coefficient (ADC) repeatability is important to body diffusion weighted imaging (DWI). However, data onto evaluating the repeatability of ADC values for normal pancreas are limited. The aim of this study was to evaluate the ADC test-retest repeatability of the normal pancreas.Methods Twenty-six healthy volunteers (mean 47.6 years; 13 men) were included and scanned twice with reposition using a DWI sequence at 3.0-T. ADCs of the pancreatic head, body and tail for two DWIs were measured by two readers. The mean ADCs of the pancreatic head, body and tail were calculated as the global pancreatic ADC. Test-retest repeatability and agreement of ADC measurement were evaluated by the Bland-Altman analysis, intra-class correlation coefficient (ICC) and coefficient of variation (CV).Results The global pancreatic ADC showed the best test-retest repeatability (mean difference ± limits of agreement were 0.05 ± 0.25×10 -3 mm 2 /s; ICC, 0.79; CV, 6%). Test-retest repeatabilities for ADC of pancreatic head, body or tail were scattered, with mean difference ± limits of agreement between two tests were 0.03 ± 0.47, 0.05 ± 0.42 and 0.06 ± 0.31 (×10 -3 mm 2 /s), respectively (ICCs, 0.81, 0.52 and 0.68; CVs, 9%, 8% and 8%). Both intra-observer repeatability and inter-observer reproducibility were acceptable for whole pancreas between ADC measurements of the two DWIs.Conclusions ADC values of the pancreas are technically challenging and repeatability is variable. Cautions should be taken when interpreting longitudinal clinical changes in ADC values of the normal pancreas because the measurements do have an inherent variability by locations.

Semiconducting nanocrystals (colloidal quantum dots, QDs) have superior optical properties and solution processibility. Silver selenide quantum dots are important nanomaterials with absorption peak extending from ~880nm to ~1200nm. Compared to the lead-based counterparts, Ag₂Se QDs are less developed and their application is less explored. This is in part due to the difficult control of their size and size distribution, resulting in broad absorption or even featureless absorption. We employed secondary phosphine to elevate the precursor reactivity and were able to well control the nucleation and growth. After a systematic investigation on a number of synthetic parameters, high quality Ag₂Se QDs have been synthesized with tunable size and narrow size distribution, exhibiting well-defined absorption peaks. The synthesis is scalable with high yield. The application of Ag₂Se QDs has been successfully demonstrated in photodetectors with photoresponse in near infrared region (>1000 nm)., 2019 IEEE International Flexible Electronics Technology Conference (IFETC), Aug. 11-14, 2019, Vancouver, BC

We report on inkjet printable gate-dielectric based on a spin-on-glass (SOG) material for applications in n-type organic field-effect transistors (OFETs). The SOG material is polymethylsilsesquioxane in alcohol mixture. After annealed at 135 °C in air, the SOG films are well crosslinked and have a good resistance against alcohol, which allows for the inkjet printing of Ag gate electrodes on top of the SOG dielectric. The crosslinked SOG films are very dense, and can withstand high electric field. This is very beneficial to the operation of transistors. In addition, the SOG films have very low hydroxyl content after annealing. This property is very important for n-type transistors. After ink formulation, this SOG dielectric has an excellent inkjet-ability with good uniformity and reproducibility. By using Polyera's P(NDI2OD-T2) as the semiconductor and SOG as the dielectric, bottom-contact top-gated n-type transistors were successfully fabricated on PET substrates with electron mobility above 0.1 cm2/V and high on/off ratio well above 105. These remarkable results demonstrate that this newly formulated SOG dielectric is a promising candidate for the future development of flexible electronic devices.

The synthesis of 4,4ʹ-difluoro-2,2ʹ-bithiophene is reported and an alternating donor-acceptor copolymer of this moiety and diketopyrrolopyrrole has been prepared. This polymer has a lower highest occupied molecular orbital than its non-fluorinated analogue polymer. Organic thin film transistors based on this polymer showed p-type charge transport behavior and a hole mobility of 0.21 cm2 V−1 s−1 in bottom-gate bottom-contact devices. Organic solar cells using this polymer as donor and [6,6]-Phenyl-C71-butyric acid methyl ester as acceptor achieved a power conversion efficiency of 3.4% with a high fill factor of 69%. Our morphology analysis showed that there was a lack of long-range ordered structure in the neat polymer thin film, which could cause the inferior device performance.

Silver nanowires, carbon nanotubes and graphenes have found applications for flexible electronics. The adhesion and the patterning of those materials on polymer substrates have been a challenge. This paper reports a novel approach for the improvement of the adhesion between silver nanowires and polyethylene terephthalate (PET) film by using a thermally crosslinkable polymer. The technique was found very efficient in improving the adhesion between silver nanowires and PET substrates.

Bulk heterojunction (BHJ) organic solar cells based on p-type polymers and n-type fullerene derivatives have attracted increasing attention due to their promising potential for providing low–cost solar electricity. In this paper, we will introduce our recent work on the development of new fullerene derivatives. Through Rh complex catalyzed coupling reactions and cycloaddition reactions, we attached several organic functional groups to the fullerene skeleton. It was found that the organic substituents had little impact on the fullerene energy levels. Instead, the organic substituents have huge impact on the other physical properties of the resulting fullerene derivatives, such as solubility, crystallinity, and electron mobility. We designed and synthesized a novel series of alkoxy substituted indene derivatives as solubilizing groups for fullerenes. Preliminary experimental results demonstrated that these 5-alkoxyindene modified C70 derivatives are superior to widely used PC71BM when blended with poly[N-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'- benzothiadiazole)](PCDTBT) in BHJ solar cells. The device series resistance decreased from 10 cm2 for the PC71BM based device to 4 cm2 for the 5-methoxyindene-C70 monoadduct based device. As a result, the device fill factor increased from 0.60 to 0.69, and the overall EQE-calibrated power conversion efficiency was enhanced from 5.6% to 6.2%., International Conference on Renewable Energies and Power Quality (ICREPQ’18), March 21-23, 2018, Salamanca, Spain

OBJECTIVE:Develop and optimize an accelerated, high-resolution (0.5mm isotropic) 3D black blood MRI technique to reduce scan time for whole-brain intracranial vessel wall imaging. MATERIALS AND METHODS:A 3D accelerated T1-weighted fast-spin-echo prototype sequence using compressed sensing (CS-SPACE) was developed at 3T. Both the acquisition [echo train length (ETL), under-sampling factor] and reconstruction parameters (regularization parameter, number of iterations) were first optimized in 5 healthy volunteers. Ten patients with a variety of intracranial vascular disease presentations (aneurysm, atherosclerosis, dissection, vasculitis) were imaged with SPACE and optimized CS-SPACE, pre and post Gd contrast. Lumen/wall area, wall-to-lumen contrast ratio (CR), enhancement ratio (ER), sharpness, and qualitative scores (1-4) by two radiologists were recorded. RESULTS:The optimized CS-SPACE protocol has ETL 60, 20% k-space under-sampling, 0.002 regularization factor with 20 iterations. In patient studies, CS-SPACE and conventional SPACE had comparable image scores both pre- (3.35±0.85 vs. 3.54±0.65, p=0.13) and post-contrast (3.72±0.58 vs. 3.53±0.57, p=0.15), but the CS-SPACE acquisition was 37% faster (6:48 vs. 10:50). CS-SPACE agreed with SPACE for lumen/wall area, ER measurements and sharpness, but marginally reduced the CR. CONCLUSION:In the evaluation of intracranial vascular disease, CS-SPACE provides a substantial reduction in scan time compared to conventional T1-weighted SPACE while maintaining good image quality.

Chest tomosynthesis is a low-dose, quasi-3D imaging modality that has been demonstrated to improve the detection sensitivity for small lung nodules, compared to 2D chest radiography. The purpose of this study is to investigate the feasibility and system requirements of stationary chest tomosynthesis (s-DCT) using a spatially distributed carbon nanotube (CNT) x-ray source array, where the projection images are collected by electronically activating individual x-ray focal spots in the source array without mechanical motion of the x-ray source, detector, or the patient. A bench-top system was constructed using an existing CNT field emission source array and a flat panel detector. The tube output, beam quality, focal spot size, system in-plane and in-depth resolution were characterized. Tomosynthesis slices of an anthropomorphic chest phantom were reconstructed for image quality assessment. All 75 CNT sources in the source array were shown to operate reliably at 80 kVp and 5 mA tube current. Source-to-source consistency in the tube current and focal spot size was observed. The incident air kerma reading per mAs was measured as 74.47 uGy mAs(-1) at 100 cm. The first half value layer of the beam was 3 mm aluminum. An average focal spot size of 2.5 × 0.5 mm was measured. The system MTF was measured to be 1.7 cycles mm(-1) along the scanning direction, and 3.4 cycles mm(-1) perpendicular to the scanning direction. As the angular coverage of 11.6°-34°, the full width at half maximum of the artifact spread function improved greatly from 9.5 to 5.2 mm. The reconstructed tomosynthesis slices clearly show airways and pulmonary vascular structures in the anthropomorphic lung phantom. The results show the CNT source array is capable of generating sufficient dose for chest tomosynthesis imaging. The results obtained so far suggest an s-DCT using a distributed CNT x-ray source array is feasible.

The mass production of organic photovoltaics requires high throughput processes capable of producing a functional active layer which is homogenous, pinhole free and of a specified thickness. Common methods to achieve this are slot-die coating and gravure printing. We have demonstrated the fabrication of poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole (PCDTBT) based photovoltaic cells using the flexographic printing technique, which enables high throughput patterned active layers to be deposited on flexible substrates at lower cost. This was achieved by optimizing the flexographic plate pattern, print speed, the solvent and the drying process. By incorporating halftone patterning, a common process in the graphics printing industry, and optimizing the printing speed, the homogeneity of the active layer print was significantly improved. Further studies of suitable solvents and drying conditions led to reduced pinhole formation and improved uniformity. The functionality of the flexographically printed active layer was demonstrated by fabrication of 1 cm2 photovoltaic cells which showed an efficiency of up to 3.5%, which is comparable to alternative deposition techniques. These results demonstrate the suitability of flexography as a fabrication technique for bulk heterojunction organic photovoltaics.

A facile method for the synthesis of highly luminescent cadmium telluride (CdTe) quantum dots (QDs) in an aqueous solution is presented. Ascorbic acid and sodium tellurite were used as the reducing agent and tellurium source, respectively, to synthesise CdTe QDs under mild conditions. In this method, the preparation of the Te precursor and the growth of CdTe QDs were achieved with a one-step synthetic route. CdTe QDs with five different emission wavelengths of 506, 517, 541, 564 and 575 nm were obtained by tuning the refluxing time. The quantum yields of these QDs were 5.4, 10.9, 37.4, 53.3 and 40.1%, respectively. The X-ray powder diffraction and transmission electron microscopy characterisation indicated that the obtained QDs had a pure cubic zinc blended structure with spherical shape. The main advantage of the present method is that both the reducing agent and tellurium precursor are stable and cheap. Furthermore, no toxic gases are released in the preparation process. It is a low cost and green method suitable for large-scale synthesis.

Taurine crystals (TC) with high fluidity, anticaking, and antisticking properties are commercially required by manufacturers. Crystal size and shape are two main factors controlling these properties. This study investigated the size and shape of TCs produced by a cooling crystallization processing. The addition of activated carbon to decolor the product exhibited negligible effects on the size of TCs. Increasing the crystallization temperature resulted in the formation of small-size crystals, whereas TC size decreased as the stirring rate increased. A careful control of post high-speed stirring aging in a slowly stirred system enabled the preparation of larger crystals. The estimation of optimal crystallization time is crucial as lengthening the crystallization time might not effectively increase the overall size of the crystals. It was found that the shape of the TC was not affected by various operating conditions, delineating a similar inherent molecular arrangement of taurine molecules under the range ...

The impact of uniaxial strain along the [111] direction on the structural and electronic properties of bulk HgTe in the zinc blende is studied by DFT. Uniaxial strain can effectively manipulate the local lattice structure along the same direction. The transformation may be caused to form the graphite-like structure in large compression and the layered one under stretching. Meanwhile, the conductive band minimum (CBM) and valence band maximum (VBM) are gradually shifted to form the indirect band structures. Further, the band gap is opened in HgTe for the significant stretching, The uniaxial compression only changes the coordination of HgTe by maintaining the semi-metallic properties.

Polymer/inorganic-nanocrystals bulk heterojunction solar cells, where inorganic semiconductor nanocrystals such as CdSe, CdS, CdTe, ZnO, TiO 2 , and silicon, replace the fullerene molecules as the electron acceptors, typically exhibit a power conversion efficiency (PCE) below 3% even after tremendous engineering efforts to optimize the nanocrystal size, shape, and nanoscale morphology. One promising feature of polymer hybrid solar cells is the ability to sensitize conjugated polymers, which on their own absorb only in the visible part of solar spectrum, into the infrared spectral range using infrared-active lead salt nanocrystal quantum dots (NQDs). Here we observed for the first time hole transfer from PbS NQDs to polymers as evidenced by the quenching of the PbS photoluminescence (PL), a sign of the presence of charge separating type II heterojunction. The type II band-offset at the NQD/polymer heterojunction enables efficient hole extraction from NQDs and leads to a record PCE of 3.80%, realized in a planar junction configuration under simulated air mass 1.5 global (AM 1.5G) irradiation of 100 mW/cm 2 . The photocurrent has an extended spectral range spanning from the ultraviolet (UV) to the infrared (IR). Contributions from the polymer and PbS to the photocurrent were identified. Infrared photons (>700 nm) contribute about 30% of the photocurrent and yield a high external quantum efficiency (EQE) of 20% at 1050 nm.

Background The relatively high radiation dose from micro-CT is a cause for concern in preclinical research involving animal subjects. Interior region-of-interest (ROI) imaging was proposed for dose reduction, but has not been experimentally applied in micro-CT. Objective Our aim is to implement interior ROI imaging in a carbon nanotube (CNT) x-ray source based micro-CT, and present the ROI image quality and radiation dose reduction for interior cardiac micro-CT imaging of a mouse heart in situ. Methods An aperture collimator was mounted at the source-side to induce a small-sized cone beam (10 mm width) at the isocenter. Interior in situ micro-CT scans were conducted on a mouse carcass and several micro-CT phantoms. A GPU-accelerated hybrid iterative reconstruction algorithm was employed for volumetric image reconstruction. Radiation dose was measured for the same system operated at the interior and global micro-CT modes. Results Visual inspection demonstrated comparable image quality between two scan modes. Quantitative evaluation demonstrated high structural similarity index (up to 0.9614) with improved contrast-noise-ratio (CNR) on interior micro-CT mode. Interior micro-CT mode yielded significant reduction (up to 83.9%) for dose length product (DLP). Conclusions This work demonstrates the applicability of using CNT x-ray source based interior micro-CT for preclinical imaging with significantly reduced radiation dose.

We have developed a clinically ready first generation stationary breast tomosynthesis system (s-DBT). In the s-DBT system, focal spot blur associated with x-ray source motion is completely eliminated, allowing for rapid acquisition of projection images over a larger angular span without changing the acquisition time. In the phantom studies the 1st generation s‐DBT system has demonstrated 30% higher spatial resolution than the corresponding continuous motion DBT systems. The system is currently being evaluated for its diagnostic performance in 100 patient clinical evaluation against FFDM. Initial results indicate that the s­‐DBT system can produce increased lesion conspicuity and comparable MC visibility. However due to x­‐ray flux limitations, certain large size patients have to be excluded. Recent studies have shown that increasing the angular span beyond 30° can be beneficial for enhanced depth resolution. We report the preliminary characterization of the 2nd generation s-­DBT system with a new CNT x-­ray source array, increased tube flux and a larger angular span. Increasing x‐ray tube flux allows for a larger patient population and dual energy imaging. Results indicate that the system delivers more than twice the flux, allowing for imaging of all size patients with acquisition time of 2­‐4 seconds. A 7° increase in angular span over 1st generation decreased the ASF by 37%. Additionally, the 2nd generation s‐DBT system utilizing a specific AFVR reconstruction method resulted in a 92% increase in the in plane resolution over CM DBT system, and a 37% increase in spatial resolution over the 1st generation s-‐DBT system.

In this work, two high boiling-point solvents (1-chloronaphthalene (CN) and 1,8-diiodooctane (DIO)) were utilized as co-additives in ortho-dichlorobenzene (ODCB) and chlorobenzene (CB) solutions to fine tune the donor and acceptor domains in the bulk heterojunction (BHJ) of poly(benzo[1,2-b:4,5-b′]dithiophene-alt-thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) and fullerene derivatives. A power conversion efficiency of 7.1% and a fill factor up to 70% were obtained for solar cells with active area of 1 cm2 when using [6,6]-phenyl C61-butyric acid methyl ester (PC60BM) as acceptor, suggesting that an optimized morphology was achieved.

A series of low-bandgap alternating copolymers of dithienosilole and thienopyrrolodione (PDTSTPDs) are prepared to investigate the effects of the polymer molecular weight and the alkyl chain length of the thienopyrrole-4,6-dione (TPD) unit on the photovoltaic performance. High-molecular-weight PDTSTPD leads to a higher hole mobility, lower device series resistance, a larger fill factor, and a higher photocurrent in PDTSTPD:[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) bulk-heterojunction solar cells. Different side-chain lengths show a significant impact on the interchain packing between polymers and affect the blend film morphology due to different solubilities. A high power conversion efficiency of 7.5% is achieved for a solar cell with a 1.0 cm2 active area, along with a maximum external quantum efficiency (EQE) of 63% in the red region.

0.2PbNi 1/3 Nb 2/3 –0.8Pb(Zr,Ti)O 3 (PNN–PZT) thick films were deposited on Pt wire with the diameter of 50 μm by electrophoretic deposition (EPD) method. The EPD deposition times on the microstructures of PNN–PZT thick films were investigated. By optimizing the EPD process, the Pt wire can be uniformly wrapped with the PNN–PZT powders. During the sintering process, the as-deposited PNN–PZT/Pt wires were buried in the mixed powders of PbCO 3 and ZrO 2 , and then sintered in the optimal temperature to get a dense microstructure. The piezoelectric properties of the thick films were characterized by scanning force microscopy (SFM) method. The results show that the PNN–PZT thick films prepared by EPD method have good piezoelectricity.

The ability to control light-matter interaction is central to several potential applications in lasing, sensing and communication. Graphene plasmons provide a way of strongly enhancing the interaction and realizing ultrathin optoelectronic devices. Here, we find that photoluminescence (PL) intensities of the graphene/GeSi quantum dots hybrid structures are saturated and quenched under positive and negative voltages at the excitation of 325 nm, respectively. A mechanism called plasmon-gating effect is proposed to reveal the PL dependence of the hybrid structures on the external electric field. On the contrary, the PL intensities at the excitation of 405 and 795 nm of the hybrid structures are quenched due to the charge transfer by tuning the Fermi level of graphene or the blocking of the excitons recombination by excitons separation effect. The results also provide an evidence for the charge transfer mechanism. The plasmon gating effect on the PL provides a new way to control the optical properties of graphene/QD hybrid structures.

Mesoporous aluminum-doped titanium dioxide (Al-TiO2) materials with high specific surface areas were prepared via a solid-state reaction route. The properties of these materials were characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), energy dispersive spectroscopy (EDS), N2 absorption-desorption, ultraviolet visible light spectroscopy (UV-Vis) and electrochemical spectroscopy. The results show that the mesoporous structure of the product with ethanol is composed of anatase laced crystal walls with amorphous grain boundaries formed gradually by degradation. Compared with those without ethanol, these samples possess larger crystallite size since ethanol decreases the pore size at higher temperature. With the increase of ethanol amount, however, the crystallite size will grow. The amorphous grain boundaries in the mesoporous material, with a large impedance and low incidental cyclic potential, are difficult to effectively degrade and the phase transformation temperature is changed from 500 to 550 °C. The growth rate of Al-TiO2 crystallites that obeys the quadratic polynomial equation may be controlled.

Rationale and Objectives Challenges remain in the imaging of the lungs of free-breathing mice. Although computed tomographic (CT) imaging is near optimal from a contrast perspective, the rapid respiration rate, limited temporal resolution, and inflexible x-ray pulse control of most micro-CT scanners limit their utility in pulmonary imaging. Carbon nanotubes (CNTs) have permitted the development of field emission cathodes, with rapid switching and precise pulse control. The goal of this study was to explore the utility of a CNT-based micro-CT system for application in quantitative pulmonary imaging. Materials and Methods Twelve CB57/B6 mice were imaged during peak inspiration and end-exhalation using the CNT micro-CT system. The respiratory trace was derived from a sensor placed underneath the abdomen of the animal. Animals were allowed to breathe freely during the imaging under isoflurane anesthesia. Images were reconstructed using isotropic voxels of 77-μm resolution (50 kVp, 400 projections, 30-ms x-ray pulse). Lung volumes were measured with region-growing techniques and thresholds derived from the surrounding air and soft tissues. Basic functional parameters, including tidal volume, functional reserve capacity and minute volume, were also calculated. Results The average scan time was 13.4 ± 1.8 minutes for each phase of the respiratory cycle. Mean lung volumes at peak inspiration and end-expiration were 0.23 ± 0.026 and 0.11 ± 0.024 mL, respectively. The average minute volume was 11.93 ± 2.64 mL/min. Conclusions The results of this study demonstrate the utility of a CNT-based micro-CT system in acquiring prospectively gated images from free-breathing mice for obtaining physiologic data. This technique provides an alternative to breath-hold techniques requiring intubation and offers greater dose efficiency than retrospective gating techniques.

Here, we present a detailed investigation on the influence of the growth conditions of colloidal lead sulfide (PbS) nanocrystals on photovoltaic device performance. The PbS nanocrystals were synthesized in a noncoordinating solvent, 1-octadecene, using oleic acid (OA) as the ligand. It was found that both the feeding molar ratio of OA to Pb and the reactant concentration were critical for controlling the growth rate of nanocrystals. Transient photocurrent (TPC) measurements revealed reduced trap density in thin films using the slow-growth nanocrystals. Solar cells based on the slow-growth nanocrystals showed a high power conversion efficiency (PCE) of 3.8% under simulated Air Mass 1.5 Global (AM 1.5G) irradiation (100 mW/cm2), a 2-fold increase in PCE, compared to the fast-growth nanocrystals, because of the remarkable improvement in the open-circuit voltage and fill factor in the PV devices.

While stable polymer electrochromic properties are mainly realized from the electrochemical oxidation of p-type conjugated polymers, in this paper we report that stable electrochromic properties can also be easily achieved by electrochemical reduction of n-type polymers, such as PF4Ox, an alternating copolymer of quater(9,9-dioctylfluorene) and oxadiazole. The introduction of the strong electron-accepting oxadiazole unit into the poly(9,9-dioctylfluorene) main chain promoted the stability of the polymer in the electrochemical reduction. The copolymer film displayed an excellent elecrochromic property upon electrochemical reduction, which turned the colorless film at the neutral state to rosy-red at the electron charged state with a switching time of 1.1 s. The film exhibited a high stability during the electrochromic test, having only 30% loss of the transmittance change in 500 potential square-wave cycles. Fourier transform infrared (FT-IR) study revealed that decay of the electrochromic performance of the film was associated with the formation of fluorenone and quinone defects on the fluorene unit in the copolymer chain. The laser flash photolysis test showed the monomer unit had a high potential to localize an electron and became highly vulnerable to be attacked by oxygen to form the defects. These defects would act as charge carrier traps and produced a pair of inner peaks at -1.84 and 1.03 V in the cyclic voltammetry test of the copolymer film.

Novel low optical gap, p-type semiconducting oligothiophenes asymmetrically end-capped with triarylamino and tricyanovinyl groups, PhN-OFOT(n)-TCN (n = 2, 3), have been synthesized and characterized for photovoltaic applications. With an incorporation of a tricyanovinyl accepting group to the triarylamine, the optical energy gap greatly reduces to 1.46 eV and the LUMO level lowers to 3.9 eV. The initial studies of the bilayer heterojunction PV cells based on the newly developed tricyanovinyl-substituted chromophores as a donor material and C60 as an acceptor material showed a PCE up to 1.33% with a large open-circuit voltage of 0.82 V in the annealed devices which makes this class of materials promising for further development. Our findings also suggest for the first time that the tricyanovinyl group is highly efficient to lower the LUMO level and reduce the optical energy gap of a p-type semiconducting photosensitizer. It is interesting to find that PhN-OFOT(2)-TCN showed better device performance in bil...

A novel photovoltaic solar assisted heat pump (PV-SAHP) system has been proposed in this study. Performance tests with a range of condenser supply water temperature were conducted on an experimental rig. The dynamic performance of this PV-SAHP system in a 4-day period with very similar weather conditions was analyzed and the influencing factors were identified. The results indicate that this PV-SAHP system has a superior coefficient of performance (COP) than the conventional heat pump system and at the same time, the photovoltaic efficiency is also higher. The COP of the heat pump was able to reach 10.4 and the average value was about 5.4. The average photovoltaic efficiency was around 13.4%. The highest overall coefficient of performance (COPp/t), bringing into consideration both the photovoltaic and thermal efficiency, was about 16.1.

Sonic infrared (SIR) imaging is a novel hybrid technology that employs ultrasonic excitation and infrared imaging to detect defects in materials. There are several parameters can affect the detectability of a defect in Sonic IR. One is the closure status of a defect, say a crack in a material. We have studied the relationship between the heating in a crack and crack closure. The crack closure was realized by applying a variety of clamping forces on the crack. In this paper, we provide quantitative results of the study on infrared signal levels at different status of crack closure. The temperature profile of a fixed point at the crack during the excitation period was presented. The thermal energy transformed from the kinetic movement was analysed over a fixed region for all clamping cases. Meanwhile, the crack energy at two fixed points crossing the crack was also studied.

Four monodisperse starburst oligomers bearing a 4,4',4"-tris(carbazol-9-yl)-triphenylamine (TCTA) core and six oligofluorene arms are synthesized and characterized. The lengths of oligofluorene arms vary from one to four fluorene units, giving the star-burst oligomers molecular weights ranging from 3072 to 10068 Da (1 Da = 1.66 x10 -27 kg). All of the starburst oligomers have good film-forming capabilities, and display bright, deep-blue fluorescence (λ max = 395-416 nm) both in solution and in the solid state, with the quantum efficiencies of the films (Φp L ) varying between 27 and 88 %. Electrochemical studies demonstrate that these materials have large energy gaps, and are stable for both p-doping and n-doping processes. Electroluminescent devices are successfully fabricated using these materials as hole-transporting emitters, and emit deep-blue light. Devices with luminance values up to 1025 cd m -2 at 11 V and luminous efficiencies of 0.47 cd A -1 at 100 cd m -2 have been produced, which translates to an external quantum efficiency of 1.4 %. In addition, these large-energy-gap starburst oligomers are good host materials for red electrophosphorescence. The luminance of the red electrophosphorescent devices is as high as 4452 cd m -2 , with a luminous efficiency of 4.31 cd A -1 at 15 mA cm -2 : This value is much higher than those obtained from the commonly used hole-transporting materials, such as poly(vinyl carbazole) (PVK) (1.10 cd A -4 at 16 mA cm -2 ).

Conventional X-ray sources face limitations due to their reliance upon thermionic emission for electron generation. A recently developed X-ray source avoids this problem by using carbon nanotubes (CNTs) as a cathode material for field emission of electrons instead of a heated tungsten filament. This CNT X-ray source is built compactly and is capable of high flux and excellent temporal resolution, and it is well suited for a variety of biomedical imaging applications. Here, we discuss the design of a micro-computed tomography system employing a CNT field-emission X-ray tube and its applications for live small-animal imaging in preclinical studies of human illness such as cancer and cardiac disease.

Intraoral dental tomosynthesis and closely related tuned-aperture CT (TACT) are low-dose three-dimensional (3D) imaging modalities that have shown improved detection of multiple dental diseases. Clinical interest in implementing these technologies waned owing to their time-consuming nature. Recently developed carbon nanotube (CNT) X-ray sources allow rapid multi-image acquisition without mechanical motion, making tomosynthesis a clinically viable technique. The objective of this investigation was to evaluate the feasibility of and produce high-quality images from a digital tomosynthesis system employing CNT X-ray technology.A test-bed stationary intraoral tomosynthesis unit was constructed using a CNT X-ray source array and a digital intraoral sensor. The source-to-image distance was modified to make the system comparable in image resolution to current two-dimensional intraoral radiography imaging systems. Anthropomorphic phantoms containing teeth with simulated and real caries lesions were imaged using a dose comparable to D-speed film dose with a rectangular collimation. Images were reconstructed and analysed.Tomosynthesis images of the phantom and teeth specimen demonstrated perceived image quality equivalent or superior to standard digital images with the added benefit of 3D information. The ability to "scroll" through slices in a buccal-lingual direction significantly improved visualization of anatomical details. In addition, the subjective visibility of dental caries was increased.Feasibility of the stationary intraoral tomosynthesis is demonstrated. The results show clinical promise and suitability for more robust observer and clinical studies.

Chest tomosynthesis is a low-dose 3-D imaging modality that has been shown to have comparable sensitivity as CT in detecting lung nodules and other lung pathologies. We have recently demonstrated the feasibility of stationary chest tomosynthesis (s-DCT) using a distributed CNT X-ray source array. The technology allows acquisition of tomographic projections without moving the X-ray source. The electronically controlled CNT x-ray source also enables physiologically gated imaging, which will minimize image blur due to the patient’s respiration motion. In this paper, we investigate the feasibility of prospective gated chest tomosynthesis using a bench-top s-DCT system with a CNT source array, a high- speed at panel detector and realistic patient respiratory signals captured using a pressure sensor. Tomosynthesis images of inflated pig lungs placed inside an anthropomorphic chest phantom were acquired at different respiration rate, with and without gating for image quality comparison. Metal beads of 2 mm diameter were placed on the pig lung for quantitative measure of the image quality. Without gating, the beads were blurred to 3:75 mm during a 3 s tomosynthesis acquisition. When gated to the end of the inhalation and exhalation phase the detected bead size reduced to 2:25 mm, much closer to the actual bead size. With gating the observed airway edges are sharper and there are more visible structural details in the lung. Our results demonstrated the feasibility of prospective gating in the s-DCT, which substantially reduces image blur associated with lung motion.

Micro-CT is a high-resolution volumetric imaging tool that provides imaging evaluations for many preclinical applications. However, the relatively high cumulative radiation dose from micro-CT scans could lead to detrimental influence on the experimental outcomes or even the damages of specimens. Interior micro-computed tomography (micro- CT) produces exact tomographic images of an interior region-of-interest (ROI) embedded within an object from truncated projection data. It holds promises for many biomedical applications with significantly reduced radiation doses. Here, we present our first implementation of an interior micro-CT system using a carbon nanotube (CNT) field-emission microfocus x-ray source. The system has two modes – interior micro-CT mode and global micro-CT mode, which is realized with a detachable x-ray beam collimator at the source side. The interior mode has an effective field-of-view (FOV) of about 10mm in diameter, while for the global mode the FOV is about 40mm in diameter. We acquired CT data in these two modes from a mouse-sized phantom, and compared the reconstructed image qualities and the associated radiation exposures. Interior ROI reconstruction was achieved by using our in-house developed reconstruction algorithm. Overall, interior micro-CT demonstrated comparable image quality to the conventional global micro-CT. Radiation doses measured by an ion chamber show that interior micro-CT yielded significant dose reduction (up to 83%).

In this paper, we report on the synthesis and properties of a novel series of multi-triarylamine-substituted carbazole-based dendrimers G2-OTP(n)-G2 (n = 2−5) with an oligothiophene core. The length of the oligothiophene core varied from 2 to 5 thiophene units. It was found that both the absorption and fluorescence emission peaks red-shifted with increasing oligothiophene core length. As a result, the power conversion efficiencies of the as-fabricated solar cells using the composite of these dendrimers and PCBM as the photoactive layer increased with increasing oligothiophene core length, from 0.04% for G2-DTP-G2 to 0.13% for G2-PTP-G2 under AM 1.5 simulated solar illumination at an irradiation intensity of 100 mW/cm2. After a thermal annealing at 150 °C for 15 min under nitrogen, however, the devices based on the G2-DTP-G2:PCBM blend exhibited the most dramatic improvement and almost best performance, although other devices also showed some improvement. Since only G2-DTP-G2 is the a crystalline material ...

We investigated the conducting properties of functionalized single wall nanotubes (SWNTs) with a finite addend concentration. Robust differences are found between monovalent and divalent additions. For the former a small number of addends can significantly disrupt the ballistic conductance of nanotubes near the Fermi level. As the concentration increases the conductance decreases rapidly and approaches zero at addend to C ratio around 25%. In contrast, divalent functionalizations have weak effects, and the nanotube quantum conductance remains above 50% of that of a perfect tube even for an addend concentration as large as 25%. These differences can be attributed to the formation of impurity states near the Fermi level for monovalent additions, while divalent addends create impurity states far away from the Fermi level.

This paper provides an analytical solution for the critical buckling stress of adhesively bonded aluminum hat sections under static axial compression. The governing rectangular plate member of the structure is treated based on the differential equation for out-of-plane deflections of thin plates. Finite element eigenvalue buckling analysis is performed to verify the assumed simply supported boundary conditions for common edges between adjacent plate elements. Elastic restraint is applied to the two loaded edges of the rectangular plate, and the relative critical buckling stress is computed according to the transcendental equations. It is found from experiments that there is no adhesive bonding failure in the elastic buckling stage. The analytical solution yields buckling stress predictions which are in reasonable agreement with measured values.

precise location and addition mode of newly attached groups have not been determined experimentally. Original articles proposed that cyclopropane-like subunits were formed by the addition of carbene [2–4] and nitrene [5, 6] to the SWCNT sidewalls. However, ab initio studies based on finite tube models favor open structures, instead of closed three-membered rings. [10] The sidewall opening has direct consequences, which are open to investigation. For instance, how does the sidewall functionalization affect the electronic structures of SWCNTs? To what extent do the changes in properties depend on the concentration of functional groups? What is the difference between the tube-addend interactions in metallic- and semiconductor-type nanotubes? We now address these questions computationally by employing density functional theory (DFT) with periodic boundary conditions (PBC).

Ab initio calculations reveal distinct electronic properties associated with functionalized single-wall carbon nanotubes (SWNTs) with monovalent sidewall additions. The metallic tubes are found to be more reactive than the semiconducting ones. The hybridization between the addend and the tube induces unique impurity states near the Fermi level. For semiconducting tubes the state is localized near the functionalized site with a characteristic length of 15 A. In contrast, the impurity state is extended and acts as a strong scattering centre in metallic tubes. This effect greatly hinders the ballistic transport of electrons along the metallic nanotubes. The characteristic dependence of the electronic states and the conductance on functional molecules provides possible pathways for chemical sensor applications and the band structure engineering.

The unique geometry of carbon nanotubes offers an ideal template for designing one-dimensional metal/nanotube hybrid structures. Through ab initio calculations we found that transition-metal/nanotube hybrid structures exhibit very interesting physical properties. The hybrid structures can have drastically different conduction properties from those of the pristine tubes and considerable magnetic moments. In some instances perfect spin polarization is achieved. The results point to a new and promising approach that uses such hybrid structures as devices for spin-polarized transport, which is expected to provide immense applications in the emerging field of spintronics.

Raman scattering measurements of single-walled carbon nanotubes prepared by laser ablation with Ni∕Co catalyst show that samples that have not been purified have a graphitic mode frequency that is 8cm−1 lower than that of samples from which most of the catalyst has been removed. The shift is attributed to charge transfer from the catalyst particles to the nanotubes. The charge transfer from the residual catalyst also affects the temperature dependence of the radial breathing mode.

We report a dynamic radiography system with a carbon nanotube based field-emission microfocus x-ray source. The system can readily generate x-ray radiation with continuous variation of temporal resolution as short as nanoseconds. Its potential applications for dynamic x-ray imaging are demonstrated. The performance characteristics of this compact and versatile system are promising for noninvasive imaging in biomedical research and industrial inspection.